[RPG] Is the Psionics handbook from 3.0 compatible with D&D 3.5

You can certainly use 3.0e and 3.5e books together. There were many changes between 3.0e and 3.5e, mainly focusing around balance issues. Unfortunately no intentional balancing was done in 3.0, and as such one CR 11 monster would be easy, and another CR 11 monster might be a lethal encounter.

The biggest individual changes are.

• Ranger changes to make them playable.
• Druid changes to make them playable.
• Monsters gain skills and feats.
• Improvements to grid based combat, including the 1.5 diagonal rule (which was removed again in 4e).
• Changes to many spells. For example TimeStop can only be used to effect the caster. Before it could be used to destroy enemies entirely.
• No class-specific skills. Skills that are marked "exclusive" in 3.0 are "trained only" cross-class skills in 3.5.

The official change guides can be found here.

13th level Humanoid that's able to evade a 18th level party? That's a decent CR gulf to span. Classes alone probably won't do it, since classing something to level 13 means it should be balanced against other 13th level creatures, though min/maxing with an emphasis on evasion might slant that.

As a magic item suggestion, give the NPC a unique Artifact of your own creation that gives them additional evasive powers; probably something that locks to their person so high level PCs can't just snatch it from them to make them powerless. If you want them to just be hard to nail down, have it grant etherealness or earth glide (if underground a lot). If you don't want the Artifact to be a loot item the PCs get afterward, you might try something from the Weapons of Legacy supplement, where the item in question is an item that has quests/requirements to use it to its full potential, and the NPC has done the quest(s) but the PCs haven't/can't.

For the Lycanthrope aspect, normally a Lycanthrope build has to use an "animal" as the base creature type. Can you have this NPC through some means be a non-animal lycan with additional abilities? Create a Lyncanthrope build for a Displacer Beast or similar hard-to-nail-down beast; then the NPCs evasive abilities are intrinsic.